The present invention relates to the structure of a liquid metal ion source which is furnished with tank means for a solid or powdery source material. More particularly, it relates to a liquid metal ion source which is well suited for the long-time continuous operation of the ion microbeam implantation.
FIGS. 1 and 2 show the typical basic setups of prior-art liquid metal ion sources. These two examples of the setups differ in a structure for holding a source material and in a heating method, and are ion sources generally adopted at present.
First, FIG. 1 is a schematic view showing a system wherein a source material 2 is held in direct contact with a heater 4 being a heating portion and is subjected to resistance heating by conducting current. An aperture 10 is provided centrally of the heater 4 which is held by two feedthrough terminals 8 attached to an insulating support 6. The tip of a needle-shaped emitter 12 is located centrally of the aperture 10 and is protruded therefrom. The source material 2 is placed around the aperture 10 so as to lie in contact with the heater 4 as well as the tip of the needle-shaped emitter 12. The source material 2 is melted by the resistance heating based on current fed from a heating power supply 14, and the melted source material wets the tip of the needle-shaped emitter 12. Under this state, a plus high voltage is applied to the needle-shaped emitter 12 by an accelerating power supply 16, and a minus high voltage with respect to the needle-shaped emitter 12 is applied to the side of an extracting electrode 20 by an extracting power supply 18. Thus, the source material 2 in the melted state wetting the tip of the needle-shaped emitter 12 is ionized by field ionization and is extracted as an ion beam 22.
On the other hand, in the setup of FIG. 2, a source material 2 is put in a container 24 such as a crucible, and the tip of a needle-shaped emitter 12 is protruded from an aperture 26 provided at the bottom of the container 24. The heating of the source material 2 is executed by electron-beam bombardment from a heater 4, to heat and melt the source material 2 near the aperture 26. Control electrodes 28a and 28b function to control the flow of the melted source material 2 on the surface of the tip of the needle-shaped emitter 12 by the use of a control power supply 30. Besides, the principle of extracting an ion beam 22 is the same as in the case of the setup example shown in FIG. 1. When the container such as crucible 24 is employed as in this example, the source material 2 heated can be prevented from vaporizing and scattering. Such ion source is described in, for example, Japanese Laid Open Utility Model Publication No. 56-123453 (1981).
These prior-art ion sources have the following problems:
(1) The whole quantity of the source material carried in the ion source needs to be always held in the melted state. Therefore, in a case where the source material is solid or powdery or where it is a material of high melting point, power consumption for heating the source material is high.
(2) In a case where the heater current is constant, the temperature of the source material rises with the decrease thereof, and changes in ion emission characteristics, such as increase in the total ion current, take place.
(3) In a case where the source material is an alloy which consists of metals of greatly different vapor pressures, the proportion of the alloy composition is changed by selective evaporation due to heating for a long time.
(4) The quantity of the source material to be carried is limited by the size of the reservoir or crucible or by an electric capacity for heating, and therefore forms one factor for determining the lifetime of the ion source.
(5) In a case where the ion emission can no longer be expected on account of the consumption of the source material, it is necessary to break the vacuum of the ion source once and to resupply the source material externally. The ion emission is temporarily ceased, the source material is resupplied, and the ion emission is restarted. Then, a long period of time is required immediately after the restart of the emission before ions are stably emitted, and the characteristics of emitted ion currents versus extracting voltages differ from those before the cessation of the ion emission. A period of time is also required for the resupplying operation.